Low-power electric lights which flash on and off find wide use as signaling devices on boats, bicycles, and road barricades. Frequently the mechanism which causes the light to blink involves a mechanical switch controlled by a bimetallic strip which alternatively heats and cools, thereby closing and opening the switch. The contacts of such switches are often subject to corrosion and pitting, which limits the useful life of the devices. Mechanical switches in flashing signal lamps which must be used out of doors are especially subject to attack by moisture and corrosion. In addition, bimetallic-strip flashers, particularly those adapted for use with low-voltage battery power sources, are so susceptible to jarring and mechanical shocks which can permanently damage flasher action or at very least permanently change the on and off times of the flasher. The on and off times of such flashers are also sensitive to changes in the ambient temperature.
Transistor circuits which periodically energize a load without using mechanical switches or relays have been known since the early days of transistor technology. FIG. 1 depicts such an electronic circuit which was disclosed in the July 1956 issue of Radio & Television News. Transistors Q1 and Q2 and NPN and PNP transistors respectively which are regeneratively interconnected to form a multivibrator circuit. A load resistor L is connected to a battery B through the collector and emitter of transistor Q1. The base of transistor Q1 is connected to the collector of transistor Q2, and the collector of transistor Q1, in addition to being connected to load L, is connected to the base of transistor Q1 across a capacitor C. The two transistors in the circuit turn on and off substantially simultaneously since transistor Q2 controls the base current of transistor Q1.
The multivibrator circuit of FIG. 1 has certain limitations, particularly if modern transistors are substituted for the older transistors recommended in the 1956 reference. The circuit has no provision for insuring that it will start to oscillate when a battery is connected to it, relying on leakage currents through the transistors for starting up. If more modern low-leakage current transistors are used, it is necessary, as is well known and in accordance with accepted procedures, to provide one or more additional current paths for turn-on currents.
A further limitation of the circuit of FIG. 1 is that the on-time of the two transistors, which is the length of time the load is energized, is strongly temperature dependent. The on-time is determined principally by the length of time for capacitor C to become charged through the forward-biased base-emitter junction of transistor Q2. The charging time is determined principally by the size of the capacitor and the magnitude of the limiting base current of transistor Q2. Since the limiting current of a forward-biased base-emitter junction of a transistor is generally strongly temperature dependent, the on-time for this curcuit is very sensitive to temperature changes.
U.S. Pat. No. 3,018,473 to Rodgers discloses a low-voltage transistorized electric light flasher in which a pair of coupled NPN and PNP transistors are employed to energize an electric lamp intermittently. The NPN transistor functions to apply a fluctuating bias to the emitter of the PNP transistor, whose collector and emitter are connected in series between the electric lamp and a battery.
The length of time which the lamp remains on in the circuit of the U.S. Pat. No. 3,018,473 is determined principally by the time it takes a charged capacitor connected directly to the base of the NPN transistor to discharge through the forward-biased base-emitter junction of the transistor. The discharge time of the capacitor and consequently the on-time of the lamp are expected to be strongly dependent on temperature for this circuit as in the case of the circuit of FIG. 1, since the discharge time depends on the limiting current through a forward-biased base-emitter junction of a transistor, which, as noted above, is temperature dependent.
The collector of the NPN transistor in the flasher circuit disclosed in the U.S. Pat. No. 3,018,473 is connected directly to the base of the PNP transistor without any interposed resistance. When the NPN transistor is rendered conducting, current flows through a resistor of relatively low resistance connected between the base and the emitter of the PNP transistor, which lowers the potential on the base of the PNP transistor relative to the emitter, rendering the PNP transistor conducting. The U.S. Pat. No. 3,018,473 teaches that in addition to providing an emitter-base bias voltage the resistor between the emitter and base of the PNP transistor serves to restrain the collector current in the NPN transistor to a relatively weak intensity.
U.S. Pat. No. 3,803,515 to Carlson discloses several electronic flasher circuits which employ two complementary transistors regeneratively interconnected through RC networks. Like the U.S. Pat. No. 3,018,473 discussed above, the U.S. Pat. No. 3,803,515 discloses a flasher circuit including a PNP transistor inserted between a lamp and a battery, an NPN transistor whose collector is connected to the base of the PNP transistor, and an RC network having only two resistors and a capacitor. The lamp is energized when the PNP transistor is rendered conductive, which occurs when the NPN transistor becomes conductive. One of the two resistors is connected in series between the emitter of the NPN transistor and the negative terminal of the battery. The U.S. Pat. No. 3,803,515 teaches that a purpose of this resistor is to limit current flowing through the collector and emitter of the NPN transistor and across the base-emitter junction of the PNP transistor. The on-time of the lamp is determined by the discharge time of the capacitor which is connected directly to the base of the NPN transistor. Unlike the two circuits discussed above, the discharge time of the capacitor is not principally determined by the limiting current of a forward-biased base-emitter junction because the resistor connected to the emitter of the transistor reduces the current across the base-emitter junction to less than its limiting value. Nonetheless the discharge time is strongly temperature dependent since it is roughly proportional to the input impedance of the transistor which, for this circuit configuration, depends on the temperature. As noted above, a strong temperature dependence of the on-time of a flasher lamp is a disadvantage for many applications.